Democrats, once as reluctant to act as those steering Sound Transit, are now on board with a House-backed plan to provide vehicle owners a modicum of relief from the effects of an excise tax hike embedded in the $54 billion expansion of light rail passed by voters last fall.

They’re locked in a stare-down with Senate Republicans who want to repeal most of what voters approved to give consumers a greater reprieve, a move that also would weaken the financial axles of the regional transit authority.

GOP Sens. Steve O’Ban, of University Place, and Dino Rossi, of Sammamish, the architects of this approach and avowed critics of the agency, say it’s necessary because lawmakers and voters got duped on the methods Sound Transit intended to use in calculating the vehicle excise tax.

They’ve been prosecuting their case in the court of public opinion for a couple of months. And, not coincidentally, it’s become a talking point for their party operatives on the streets of the 45th Legislative District, where a special election for a state Senate seat will be held this fall. That seat is in Republican hands now, but if Democrats win they’ll take back control of the Senate.

This week, those two Republican senators got good news as their request for a legislative inquisition of the financing calculus of Sound Transit 3 had been granted. The Senate Law and Justice Committee will hold an “investigatory work session” on a date to be announced.

One question this panel may want to drill down on is whether lawmakers read, and understood, the fine print of the 2015 legislation they approved that laid the track upon which the transit authority traveled to the ballot in 2016.

O’Ban, an attorney for nearly three decades, voted for the bill. He contends the wording obfuscated Sound Transit’s intention to use a 1990s era depreciation schedule that overvalues vehicles to calculate the excise tax levy. The Department of Licensing updated its depreciation schedule in 2006 to more accurately trace a vehicle’s declining value but the 2015 legislation doesn’t require Sound Transit use that schedule until 2028.

When the Law and Justice Committee puts this matter on trial, so to speak, Sen. Doug Ericksen, R-Ferndale, should be the first witness. He can testify why he drafted Amendment 53 to Senate Bill 5987, the legislation containing the go-ahead for Sound Transit.

His amendment axed use of the old valuation schedule and required use of a new one he crafted that depreciated cars at an even faster rate than that 2006 schedule. There’s a one-paragraph summation of the amendment’s effect, laying out in plain language what it would do.

“I think this would go a long ways toward helping people accept this type of tax,” Ericksen said on the Senate floor on Feb. 27, 2015.

His Republican colleagues didn’t back his play and the amendment failed.

Fast forward to the present.

Lawmakers are in a special session with budget writers trying to fashion a two-year state spending plan that amply funds public schools. Talks are creeping along. The Legislature is destined for a second 30-day special session beginning as early as May 24.

Everyone understands the budget is the primary task. Democratic lawmakers presumed a deal on car tabs would be part of the end game given the attention it’s received.

But there’s been no serious negotiations on a compromise car tab bill. Now, with a looming inquest in the Senate, some Democrats question if their GOP friends actually want to reach an understanding on this issue before the November election.

That might explain why directors of Sound Transit are content to let it play out. They can stomach, barely, the House plan. They can’t stand the Senate approach at all.